The present invention relates to a method and apparatus for predicting the occurrence of an earthquake, and particularly for locating and identifying electromagnetic precursors to earthquakes.
Earthquake prediction is a very practical topic which can potentially save the lives of millions of persons. Considerable efforts have been made to identify suitable earthquake precursors which may be used for predicting the occurrence of an earthquake, but nevertheless reliable prediction methods are still not completely satisfactory.
The present state of knowledge is that many regions for potential earthquakes are known, but the time of occurrence of any earthquake within these regions is still undetermined. Many precursory phenomena have been observed (e.g., seismicity; electric, magnetic, gravity anomalies; animal movements), but none has yet been found to be a reliable predictor of an earthquake. At the present time, the best method of prediction is statistical, where the probability of an earthquake occurring in a certain region over a certain number of years is given. Earthquake regions which have not had relatively recent events are of higher probability as compared to regions which have been seriously skaken recently.
About two years ago attention became focussed on electromagnetic precursors when immediately before the Oct. 17, 1989 Loma Prieta earthquake near San Francisco, ULF (ultralow frequency) magnetic noise anomalies were found to have been measured about 7 km from the epicentre. A theory, more particularly described below, has been proposed by the inventor of the present method which predicted the anomalies to a relatively high accuracy.
If the anomalies lie in the narrow regions according to this theory, as described more particularly below their use as forewarners of oncoming earthquakes becomes a very difficult process. The Loma Prieta ULF anomalies were observed using a solenoid. If such solenoids were to be used to identify the above-mentioned narrow regions, they would have to be placed at spacings at 100 meters in lines across earthquake regions like California. For these lines to be about 15 km apart, it would require about 60,000 solenoids to cover the California earthquake region.
It has been previously proposed, for example as described in USSR Patent SU1315917 (1987), to monitor variations in the vertical component of the Earth's magnetic field in order to predict the occurrence of an earthquake. In the method described in that patent, a point of observation is selected in a region having a weakly-pronounced sedimentary strata, which represents a known potential earthquake fault. At a distance of one-tenth of the seismic wavelength of the known potential earthquake fault, observations are carried out to detect the ratio of the vertical component of the pulsations of the Earth's magnetic field to its horizontal component. Taking 5 Hz as the peak frequency, and 6.5 km/sec a typical seismic velocity for bedrock (see line 4, Page 1238 in Eberhart-Phillips D. et al., Geophys. Res. Lett. 17(8) 1235-1238 (7/1990), it will be seen that one wavelength is about 1,300 meters, and therefore one-tenth wavelength is about 130 meters.
Thus, a serious limitation in the method described in this publication is that the potential earthquake fault must be known within a distance of tens of meters, since the observation (e.g., location of the magnetometer) must be at a distance of approximately 130 meters from the known potential earthquake fault. Moreover, as will be described below (particularly with reference to FIG. 3 of the accompanying drawings), if the vertical component of the magnetic field is measured too near the earthquake fault, an anomalous field will not be measured, and therefore a potential earthquake warning may be missed.